Eldarion, son of Elessar Telcontar (Aragorn II), 1st King of the Reunited Kingdom, and Arwen Undómiel, daughter of Elrond of Rivendell and Celebrían of Lórien, is the 2nd King of the Reunited Kingdom of Arnor and Gondor:

The son of Elessar and Arwen was Eldarion, first king of the Fourth Age....

The Peoples of Middle-earth, HoME Vol 12, Part 1, Ch 7, The Heirs of Elendil: The Southern Line of Gondor: The Anárioni

Eldarion had at least two sisters, whom Tolkien did not name:

Then [Arwen] said farewell to Eldarion, and to her daughters....

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers: The Tale of Aragorn and Arwen

He acceded to the throne upon the death of his father in Fourth Age 120:

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers: The Tale of Aragorn and Arwen

Tolkien estimated that his reign lasted approximately a century:

I have written nothing beyond the first few years of the Fourth Age. (Except the beginning of a tale supposed to refer to the end of the reign of Eldarion about 100 years after the death of Aragorn. Then I of course discovered that the King's Peace would contain no tales worth recounting; and his wars would have little interest after the overthrow of Sauron; but that almost certainly a restlessness would appear about then, owing to the (it seems) inevitable boredom of Men with the good: there would be secret societies practicing dark cults, and 'orc-cults' among adolescents.)

The Peoples of Middle-earth, HoME Vol 12, Part 1, Ch 7, The Heirs of Elendil: The Southern Line of Gondor: The Anárioni

Of Eldarion son of Elessar it was foretold that he should rule a great realm, and that it should endure for a hundred generations of Men after him, that is until a new age brought in again new things; and from him should come the kings of many realms in long days after. But if this foretelling spoke truly, none now can say, for Gondor and Arnor are no more; and even the chronicles of the House of Elessar and all their deeds and glory are lost.

The Peoples of Middle-earth, HoME Vol 12, Part 1, Ch 8, The Tale of Years of the Third Age